Paris.
That's the line that matters for a European team. Vercel Sandbox moved beyond Washington and went global, for a start in four cities.
Fact
The facts: Vercel Sandbox now runs in four regions, that is Washington (iad1), San Francisco (sfo1), Cleveland (cle1) and Paris (cdg1). Washington stays the default. Every plan gets region selection; only Pro and Enterprise can set a fallback region on failure. The region is set through the SDK, the command line, or the project settings. Source: Vercel's changelog.
For a team running an agent close to its own reader, the difference is between a fast answer and one unnecessary ocean crossing on every request.
One limit is worth knowing up front. A snapshot stays in the region where it was made, and doesn't move. The snapshot and the sandbox have to be in the same region, or it won't start. If you're planning Paris, plan it from the start.
Official primary sources
Vercel Changelog: Sandbox is now globally availableThe visual is generated code-art, no third-party images.